1. Field of the Invention
The present invention relates to an electron-emitting element emitting electrons utilizing the phenomenon of electric field electron emission, and to an image display device using such an electron-emitting element. More particularly, the present invention relates to a thin image display device used, for example, for audio-visual equipment.
2. Description of the Prior Art
Until now, mainly cathode ray tubes (CRTs) are used for displays (image display devices) such as color televisions or computer monitors. However recently, there is a need for ever smaller, lighter and thinner image display devices, and the development of new thinner image display devices is flourishing.
This situation has led to research and development activities for several types of thin image display devices, and of these, the development of liquid crystal displays and plasma displays is particularly thriving. Liquid crystal displays find application in many products, such as portable computers, portable televisions, video cameras, and car navigation systems. Plasma displays find application in products such as 20-inch or 40-inch large displays.
However, liquid crystal displays have the problem that their viewing angle is narrow, and their response times are slow, whereas plasma display devices have the problem that they can hardly attain high brilliance and their power consumption is large. As thin image display devices that solve these problems, image display devices utilizing the so-called xe2x80x9cfield emissionxe2x80x9d phenomenon, whereby electrons are emitted at regular temperatures in a vacuum, have had wide-spread attention (such devices are referred to as xe2x80x9cFEDsxe2x80x9d in the following). Such an FED is self-emitting, so that a broad viewing angle and a high brilliance can be attained. Moreover, its basic principle (using an electron beam to cause a phosphor to emit light) is the same as in conventional cathode ray tubes, so that an image with high color repeatability can be displayed naturally.
As the electron-emitting elements for the FED, Spindt-type microchip-type electron-emitting elements, surface-conducting elements formed on a metal thin film or an oxide thin film, and MIM-type (or similarly structured) electron-emitting elements have been proposed for example.
In recent years, carbon-based materials, such as diamond, graphite, DLC (diamond-like carbon), and carbon nanotubes have gained wide-spread attention as electron-emitting materials for making electron-emitting elements.
Such an electron-emitting element is disclosed, for example, in Japanese Patent Applications Tokkai Hei 10-149760 and Tokkai Hei 10-12124.
FIGS. 8 and 9 are schematic cross-sectional drawings of a first conventional electron-emitting element (see Tokkai Hei 10-149760). The electron-emitting element in FIG. 8 is made be applying purified carbon nanotubes 101 made by arc emission to a support substrate 102 made of a synthetic resin (FIG. 8A), and then applying a resist and forming a pattern in accordance with the layout of the electron-emitting portions 103 by lithography, so that electron-emitting portions 103 made of carbon nanotubes 101 are formed on the support substrate 102 (FIG. 8B). In this case, the carbon nanotubes 101 on the support substrate 102 lie one upon another like fallen trees, as shown in FIG. 9.
When an electric field is applied to the carbon nanotubes 101 patterned into an electron-emitting portion 103 in such an electron-emitting element, electrons are emitted by the carbon nanotubes 101.
FIG. 10 is a schematic cross-sectional drawing of a second conventional electron-emitting element (see same Tokkai Hei 10-149760). The electron-emitting element in FIG. 10 includes a support substrate 111, a cathode wiring layer 112 disposed on the support substrate 111, and an electron-emitting portion 116 disposed on the cathode wiring layer 112. The electron-emitting portion 116 includes a conductive convex portion 114 formed in a portion of a conductive material layer 113, and a plurality of carbon nanotubes 115 partially buried in the tip of the conductive convex portion 114.
The following is an explanation of a method for manufacturing this second conventional electron-emitting element. First, a substrate of a silicon single-crystal is prepared, and a female mold substrate for the conductive convex portion 114 is formed by anisotropic etching. Carbon nanotubes 115 are disposed in this concave portion, a conductive material such as tungsten is deposited on top of it by sputtering, and a conductive material for wiring is sputtered on top of that. Then, the female mold substrate is removed, resulting in an electron-emitting element as shown in FIG. 10.
In this electron-emitting element, the electron-emitting carbon nanotubes 115 are arranged at the tip of the conductive convex portion 114, where an electric field tends to concentrate, so that a large electric field can be generated with a small driving voltage, and the electron-emitting carbon nanotubes 115 emit electrons efficiently.
FIG. 11 is a schematic cross-sectional drawing of a third conventional electron-emitting element (Tokkai Hei 10-12124). The electron-emitting element shown in FIG. 11 is formed as follows. First, an aluminum film 122 is formed by, for example, vapor deposition on a flat glass substrate 121. Then, the aluminum film 122 is rinsed, and an insulating film 123 is formed by an anode oxidation process. After this process, the bottom portion of the pores 124 formed during the anode oxidation process are etched all the way to the aluminum film 122 by anisotropic RIE etching. In an electrocoloring process, a nickel metal catalyst 125 is buried in the pores 124, and successively a heating process is performed at 1150xc2x0 C. in a mixed atmosphere of methane gas and hydrogen to generate and grow carbon nanotubes 126. With these steps, electron-emitting carbon nanotubes 126 can be orientationally aligned with high precision, and arranged to form an electron-emitting element with sharp tips.
With such an electron-emitting element, the carbon nanotubes 126 can be orientationally aligned into a shape with sharp tips, so that an electric field can be concentrated effectively at the electron-emitting material to attain an electron-emitting element with high efficiency.
However, the conventional electron-emitting element shown in FIG. 8 has the following problems. First of all, if carbon nanotubes 101 are applied to the support substrate 102 by a method such as printing, since the carbon nanotubes 101 have a rod-like longish molecular shape, they lie one upon another like fallen trees, as shown in FIG. 9, when attached to the support substrate 102. In this situation, the orientation of the electron-emitting carbon nanotubes 101 poses the problem that the tips, which are the most important for the electron emission, are partially buried. Thus, when a voltage is applied to the electron-emitting element, the electric field does not concentrate effectively, so that an efficient electron-emitting element is not attained. Moreover, forcing the carbon nanotubes 101 to assume an upright position with respect to the support substrate 102, by press-inserting or burying it is extraordinarily difficult to let each and every molecule of the countless carbon nanotubes 101 stand upright on the support substrate 102.
Moreover, in the conventional electron-emitting element shown in FIG. 10, with the manufacturing method described above, the carbon nanotubes 115 are arranged on a concave portion and a conductive material is sputtered on top of them, so that the carbon nanotubes 115 are buried inside the tip of the conductive convex portion 114. Thus, even when the electric field concentrates in the conductive convex portion 114, the electric field does not sufficiently concentrate on the carbon nanotubes 115 themselves, so that an effective electron-emitting element is not attained. Moreover, the step for forming the conductive convex portion 114 itself is complicated, and there is a limit to the size of the silicon substrate, so that this is not a process that can be performed inexpensively for large substrates. Moreover, it is difficult to suppress irregularities of the convex shape, and there are problems with regard to reliability and production cost.
Moreover, in the conventional electron-emitting element shown in FIG. 11, the carbon nanotubes 126 stand upright on the glass substrate 121, so that an electron emission due to an electric field concentration can be attained. However, since the source for this electron emission is buried in the insulating film 123, when the electron emission begins, the surface of the insulating film 123 starts to charge electrically, which changes the electric field, so that emission of electrons becomes unstable and an effective electron emission is not attained. Moreover, it is also possible to let the carbon nanotubes 126 protrude somewhat from the pores 124, but it is difficult to control the protrusion amount of the carbon nanotubes 126 during the step of molecule growth, and large variations occur easily. Moreover, since this step is performed at temperatures above 1000xc2x0 C., there is the problem that regular glass sheets do not withstand such high temperatures, so that there are limitations with regard to the material and the size of the substrates, which leads to the same problems with regard to efficiency, reliability and costs as above.
With the aforementioned problems of the prior art in mind, and in consideration of favorable properties when being used for image display devices in particular, it is an object of the present invention to provide a carbon ink containing an electron-emitting component and an electron-emitting element, to which the carbon ink is applied in an inexpensive printing step suitable for mass production, the electron-emitting element having good electric field emission efficiency, being driven with low driving voltages, and being able to emit electrons even at a low degree of vacuum. It is a further object of the present invention to provide a method for manufacturing such an electron-emitting element. It is also an object of the present invention to provide an improved electron-emitting element used in an image display device.
It is another object of the present invention to provide a high-resolution image display device with high image quality and efficiency including such an improved electron-emitting element.
To realize these objects, an inventive carbon particle composition is provided. One embodiment of the present invention is an inventive carbon ink made into a paste with an organic binder and a solvent comprises carbon particles having a 6-membered carbon ring, and support particles for supporting the carbon particles.
An electron-emitting element in accordance with the present invention is made by applying, to predetermined positions of a conductor patterned onto a substrate, a carbon ink made into a paste with an organic binder and a solvent, the ink comprising (i) carbon particles having a 6-membered carbon ring, and (ii) support particles for supporting the carbon particles, and firing the ink. When the ink is applied to a substrate, the support particles support some of the carbon particles so as to stand in a substantially upright orientation with respect to the substrate. In this situation, the support particles contact the carbon particles only at portions of the surface of the carbon particles.
With the carbon ink and the electron-emitting element in accordance with the present invention, carbon particles having a 6-membered carbon ring and serving as an electron-emitting material are made into a paste with an organic binder and a solvent. Thus, they can be applied in specified positions and scope on a substrate or the cathode wiring, using an inexpensive method suitable for mass production, such as printing. Since, in this configuration, support particles are included that support the carbon particles, the carbon particles are supported by the support particles, so that they do not fall down on the substrate and lie one on top of the other, and many carbon particles are in an upright orientation with respect to the substrate. Moreover, by firing the ink after applying it, the organic binder and the solvent decompose, and carbon particles remain adhering to the conductor on the substrate. Moreover, the carbon material including 6-membered carbon rings is a good electrical conductor, and its work function is low, so that if in a vacuum environment an electric field is applied to the carbon particles of the electron-emitting element in accordance with the present invention, the carbon particles emit electrons along the force lines of the electric field. Since many carbon particles are in an upright orientation with respect to the substrate surface, an electric field tends to concentrate at the edges of the individual carbon particles, so that the emission of numerous electrons can be achieved with a weaker electric field, i.e. a lower driving voltage. Consequently, in accordance with the present invention, an electron-emitting element with high electron emission efficiency can be obtained with an inexpensive process suitable for mass production, such as printing, so that an electron-emitting element is obtained that can be suitably used for an image display device.
In the carbon ink and the electron-emitting element in accordance with the present invention, it is preferable that the size of the support particles on the substrate is smaller than a longitudinal length of the carbon particles. With this configuration, after the carbon ink has been applied to, for example, the substrate, the carbon particles enclose the support particles, but because their longitudinal length is longer than the size of the support particles, they do not form a film on the support particles, so that it can be ensured with better reliability that numerous carbon particles stand in an upright orientation on the substrate to which they have been applied. Consequently, in accordance with the present invention, it can be ensured with high reliability that an electron-emitting element is obtained, having electron-emitting portions with high efficiency, so that an electron-emitting element suitable for an image display device can be obtained.
In the carbon ink and the electron-emitting element in accordance with the present invention, it is preferable that the support particles are selected from the group consisting of a self-combustible powder that decomposes into a gas when heated or burned, and a thermally decomposing foaming agent powder. With this configuration, the ink is fired after it has been applied, and not only the organic binder and the solvent, but also the support particles are decomposed, so that only the carbon particles remain adhering to the conductor of the substrate in an upright orientation with respect to the substrate. The carbon material having a 6-membered carbon ring is a good electrical conductor, the potential of the conductor spreads through the entire aggregation of carbon particles, and the electric field reaches even the voids after the support particles have been decomposed, so that the electron emission efficiency can be improved even further.
In the carbon ink and the electron-emitting element in accordance with the present invention, it is preferable that decomposition temperature of the combustible powder and the decomposition temperature of the thermally decomposing foaming agent powder is lower than the decomposition temperature of the organic binder. With this configuration, the combustible powder or the thermally decomposing foaming agent powder are decomposed first, while the organic binder maintains numerous carbon particles in an upright orientation with respect to the substrate during the firing after the carbon ink is applied. Thus, the condition where numerous carbon particles stand in an upright orientation with respect to the substrate to which they are applied can be realized with better reliability. Consequently, in accordance with the present invention, it can be ensured with better reliability that an electron-emitting element is obtained, having electron-emitting portions with high efficiency, so that an electron-emitting element suitable for an image display device can be obtained.
In the carbon ink and the electron-emitting element in accordance with the present invention, it is preferable that, in an aggregation of carbon particles, voids having a size in the range of 0.05 to 5 xcexcm have been formed by decomposing the support particles. With this configuration, the carbon particles form aggregations enclosing voids of sizes in this range, so that even more carbon particles stand in an upright orientation on the substrate, and the influence of the electric field extends into the inside of the aggregations, which improves the electron emission efficiency even further.
In the carbon ink and the electron-emitting element in accordance with the present invention, it is preferable that the carbon particles include carbon nanotubes. With this configuration, the carbon nanotubes do not only have a high electron emission efficiency due to their longish, rod-like molecule shape, but in conjunction with the present invention, many carbon nanotubes are supported by the support particles, and assume an upright orientation with respect to the substrate to which they adhere, so that electric fields tend to concentrate even better at the tip of the carbon nanotubes, which leads to an even higher efficiency. Consequently, in accordance with the present invention, it can be ensured with better reliability that an electron-emitting element is obtained, having electron-emitting portions with high efficiency, so that an electron-emitting element suitable for an image display device can be obtained.
In the carbon ink and the electron-emitting element in accordance with the present invention, it is preferable that the carbon particles include graphite. Graphite is an inexpensive material that is easy to obtain industrially, but its efficiency of electron emission in an electric field is not as high as that of carbon nanotubes. However, by combining it with the present invention, numerous graphite particles are supported by the support particles, and can be adhered in an upright orientation with respect to the substrate to which they have been applied, so that electric fields tend to concentrate at the tip of the graphite crystals, which leads to a higher electron emission efficiency. Consequently, in accordance with the present invention, it can be ensured with better reliability that an electron-emitting element is obtained, having electron-emitting portions with high efficiency, so that an electron-emitting element suitable for an image display device can be obtained.
Moreover, if carbon fibers made into graphite powder are used for the carbon particles of the present invention, the directionality of the carbon particle clusters is improved, which has the effect of enhancing the efficiency.
A method for manufacturing an electron-emitting element in accordance with the present invention comprises filling the above-described carbon ink at least into a patterned concave board; transferring the carbon ink filled into the patterned concave board to a blanket; and transferring the carbon ink transferred to the blanket to a substrate. With this configuration, the carbon ink can be transferred and applied reliably on the substrate or the cathode wiring, even when carbon particles, support particles and organic binder with different particle sizes are mixed in the carbon ink. Moreover, numerous carbon particles are applied in an upright orientation with respect to the substrate, i.e. in the same condition in which the carbon ink is filled in the concave board. As a result, in accordance with the manufacturing method of the present invention, an electron-emitting element having electron-emitting portions with high efficiency is obtained in an inexpensive process suitable for mass production, so that an electron-emitting element suitable for an image display device can be obtained.
Using the carbon ink and the electron-emitting element of the present invention, it is possible to make an image display device. In a first configuration, an image display device for forming images by causing a phosphor layer to emit light with electrons emitted from electron-emitting elements includes a vacuum container; a phosphor layer; a substrate provided with a cathode wiring made of a patterned conductor; and electron-emitting elements made by applying, to predetermined positions of the substrate, a carbon ink made into a paste with an organic binder and a solvent, the ink comprising (i) carbon particles having a 6-membered carbon ring, and (ii) support particles for supporting the carbon particles, and firing the ink. The cathode wiring is patterned into stripes. The phosphor layer has electrically separated stripes that are arranged in a plane parallel to the stripes of the cathode wiring and extend substantially perpendicular to the stripes of the cathode wiring. The image display device is matrix-driven between the stripes of the phosphor layer and the stripes of the cathode wiring. With this configuration of an image display device, forming electron-emitting elements at the matrix intersections formed by the stripes of the cathode wiring and the stripes of the phosphor layer, and temporally changing the potential of each of the stripes in accordance with an image to be displayed causes electrons to be emitted only from electron-emitting elements at the intersections where the electric field between the stripes exceeds a value at which the electron-emitting element emits electrons, whereby the phosphor layer temporally emits light only at predetermined portions, so that, as a result, an image can be displayed. The electron-emitting element of the present invention can be manufactured in a process that is inexpensive and suitable for mass production, and its efficiency is high, so that the image display device configured as described above similarly can be manufactured in a process that is inexpensive and suitable for mass production, and has high efficiency.
In a second configuration, an image display device for forming images by causing a phosphor layer to emit light with electrons emitted from electron-emitting elements includes a vacuum container; a phosphor layer; a substrate provided with a cathode wiring made of a patterned conductor; electron-emitting elements made by applying, to predetermined positions of the substrate, a carbon ink made into a paste with an organic binder and a solvent, the ink comprising (i) carbon particles having a 6-membered carbon ring, and (ii) support particles for supporting the carbon particles, and firing the ink; and gate electrodes arranged between the phosphor layer and the substrate. The cathode wiring is patterned into stripes. The gate electrodes have electrically separated stripes that are arranged in a plane parallel to the stripes of the cathode wiring and extend substantially perpendicular to the stripes of the cathode wiring. The image display device is matrix-driven between the stripes of the phosphor layer and the stripes of the cathode wiring. With this configuration of an image display device, forming electron-emitting elements at the matrix intersections formed by the stripes of the cathode wiring and the stripes of the gate electrodes, and temporally changing the potential of each of the stripes in accordance with an image to be displayed causes electrons to be emitted only from electron-emitting elements at the intersections where the electric field between the stripes exceeds a value at which the electron-emitting element emits electrons, whereby the phosphor layer temporally emits light only at predetermined portions, so that, as a result, an image can be displayed. The electron-emitting element of the present invention can be manufactured in a process that is inexpensive and suitable for mass production, and its efficiency is high, so that the image display device configured as described above similarly can be manufactured in a process that is inexpensive and suitable for mass production, and has high efficiency.
It is preferable that the image display device according to the present invention further comprises control electrodes between the phosphor layer and the gate electrodes, the control electrodes functioning to focus or to focus and deflect an electron beam. The electron-emitting element of the present invention has a very high electron emission efficiency, so that its application area can be small, and since it can be patterned in a printing step, when it is applied at the matrix intersections, it can be regarded as a dot in comparison to the size of the pixels of the phosphor layer. Applying to this configuration the principle that light from one point can be focussed by simple optical means on one point, the focussing operation of the control electrode plate focuses the electron beam emitted by one point on one point on the phosphor layer corresponding to the electron-optical image plane, within the scope of aberrations. Thus, the spot size on the phosphor layer can be reduced, thereby attaining an image display device with a higher image resolution. Moreover, if this configuration is further provided with a deflection function, such a deflection function can scan a small focused spot over a plurality of phosphor pixels and let these phosphor pixels emit light, improving the resolution even further. Thus, with this configuration, an image display device can be obtained, that can be manufactured in an inexpensive process suitable for mass production, that has high efficiency and improved resolution.
In the image display device of the present invention, it is preferable that the substrate is integrated into the vacuum container. Combining the substrate and the vacuum container into one member, reduces material costs and facilitates the assembly process, which makes the image display device even less expensive.